In SMPSs (SMPS=Switched Mode Power Semiconductor or switched mode power supply) with MOSFETs, the switch-off thereof gives rise to high-frequency output oscillations which bring about interference and thus adversely affect the interference spectrum. This applies in particular to SMPSs which use compensation components, namely so-called CoolMOS-FETs, as MOSFETs.
Compared with SMPSs with CoolMOS-FETs, corresponding circuits with IGBTs are distinguished by a lower-interference switching behavior in which significantly fewer high-frequency output oscillations arise during switch-off.
FIG. 4 shows a MOSFET T, which is located with its source-gate path and a load L in series between a voltage source +U and a reference-ground potential and is driven at its gate G. If this MOSFET T is switched off at an instant t0, then a current I through the MOSFET falls steeply, while an output voltage Uout rises suddenly and exhibits high-frequency output oscillations especially in the case of a CoolMOS-FET, as is illustrated in FIG. 5.
By contrast, during switch-off an IGBT used instead of the MOSFET T supplies a so-called tail current Itail, which delays the fall in the current I after switch-off at the instant t0 and thus attenuates output oscillations of the output voltage Uout, so that the interference spectrum is adversely affected to a lesser extent in comparison with a CoolMOS-FET (cf. FIG. 6).
It is an object of the present invention to provide a MOSFET circuit, in particular for a switched mode power supply, which is distinguished by reduced output voltage oscillations during a switch-off operation, which is also intended to hold true when CoolMOS-FETs are used for the MOSFET circuit.